Technical Field
The present invention relates to a semiconductor device. In particular, the present invention relates to a semiconductor device configured to prevent a current from flowing when a DC power supply is reversely connected.
Background Art
A description is now given of a related-art electronic device having a function of protecting an internal circuit against so-called reverse connection in which a DC power supply such as a battery is reversely connected. FIG. 5 is a circuit diagram illustrating an electronic device disclosed in Patent Literature 1. Operations of a circuit having a protection function are described for cases where a power supply is normally and reversely connected to the electronic device.
<Normal Connection State in which Power Supply is Normally Connected to Electronic Device>
In a normal connection state in which a power supply 1 is normally connected to an electronic device 3, a voltage between a power supply terminal TV and a ground line LG is divided by resistors R1 and R2. The divided voltage is applied to a gate of an NMOS transistor MT1. When the power supply 1 is normally connected, the NMOS transistor MT1 is turned on, and accordingly current flows through a channel of the NMOS transistor MT1 to electrically connect a ground terminal TG and the ground line LG to each other, to thereby establish the supply of power to the electronic device 3.
<Reverse Connection State in which Power Supply is Reversely Connected to Electronic Device>
In a reverse connection state in which the power supply 1 is reversely connected to the electronic device 3, the voltage applied to the gate of the NMOS transistor MT1 is lower than a voltage applied to a source thereof, and accordingly the NMOS transistor MT1 is turned off, and a parasitic diode is biased in the reverse direction, with the result that the ground terminal TG and the ground line LG are not electrically connected to each other. Thus, a reverse current does not flow, and hence the electronic device 3 is protected against the reverse connection.